Considerable performance gains have been achieved lately in radio systems by using Interference Rejection Combining (IRC) receivers. The desired signal is impaired by interference. However, when the receiver equipment has two antennas, it is possible to resist interference effectively without increasing the complexity of receivers significantly. Therefore, radio systems may employ different diversity methods to increase the capacity of the system. One of them is spatial diversity, which is obtained using an array antenna comprising a plural number of antenna elements that are physically separate from each other. The received signals are combined in diversity receivers using a suitable combining method.
Current receivers are based on statistical signal models the accuracy of which cannot be relied on in all situations. A known combining method that can reduce the impact of noise and interference is the Maximal Ratio Combining (MRC) method. However, this method supposes that the interference and noise in each antenna element are independent of other antenna elements, i.e. they are white. This is not always true in actual cellular radio networks, in particular. Another known combining method is Interference Rejection Combining (IRC). IRC receivers have a good performance in radio links in which the performance is limited only by interference. The problem of the advanced IRC receivers is the more noise limited cases (sensitivity) and also the cases where there are both noise and interference present. It can be seen that the more advanced IRC receivers are used, the more there is loss in the sensitivity. The IRC receivers trust their operation to the estimated interference covariance matrix, and as it is estimated, it is not perfect. Thus, sensitivity loss compared to reference MRC receivers can be seen.
In typical operating conditions when a terminal device is on the edge of a radio cell, the radio link has a poor signal-to-noise ratio, SNR, and a high probability exists for a radio link, which is interfered by the interference from neighbouring cells. Therefore, the IRC algorithms would be highly beneficial in situations where the SNR is also low.
A typical solution has been to select the receiver algorithm based on estimation whether the case is noise or interference limited. Clearly those kinds of algorithms are not very effective. In U.S. patent application Ser. No. 10/662,826, a “soft switch” algorithm was proposed in which a compromise was made between noise and interference limited cases. The effect of hard selection was reduced but basically this algorithm makes a compromise with the performance of the receiver in different operating scenarios, i.e., it tries to reduce the loss of sensitivity by reducing some receiver performance against interference. When studying the performance of IRC receivers against interference where the two exemplary solutions of IRC algorithms are used, called here Space-Time IRC (STIRC) and Space-Time-Complex Plane IRC (STCIRC), the latter being a further development of STIRC, it can be seen that the solutions behave as compromises in both extreme cases.
Receiver sensitivity is a key performance criterion in network planning. Good base station sensitivity can allow lower mobile station transmission power, thereby reducing overall interference, allowing a longer mobile station battery life, and hence lowering the number of sub-cells in coverage limited rural areas. Therefore, one of the issues in finding a good receiver solution is to obtain an algorithm that maintains the existing sensitivity performance but does not significantly affect the interference performance. As the modelling of the interference is getting more accurate in future receivers, also the losses are getting larger, if the problems related to the receivers are not solved.